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Hurricane Miriam Discussion Number  24
NWS Central Pacific Hurricane Center Honolulu HI   EP152018
500 PM HST Fri Aug 31 2018
 
Vertical wind shear has begun to take its toll on Miriam. The eye is
no longer present in geostationary satellite imagery, and a
decreasing amount of deep convection has become confined to
the northern semicircle. Subjective Dvorak current intensity
estimates remained at 5.0/90 kt from all fix agencies. However, the
final T numbers were down to 4.0 or 4.5, and given the degradation
of the satellite presentation, the initial intensity will be lowered
to 80 kt. Also, a minor adjustment was made to the wind radii based
on partial ASCAT passes from this morning. 
 
The initial motion estimate is slightly east of due north (010 
degrees) at 10 kt. Timely microwave passes (SSMI at 0014Z and SSMIS
at 0106Z) were helpful in placing the center and determining the
short-term motion. Through tonight, Miriam will continue to be
steered northward by a mid to upper level trough to the northwest
and a deep ridge to the northeast. On Saturday, Miriam will begin a
turn toward the northwest. This will occur as the steering flow of
the weakening system becomes increasingly influenced by the low
level ridge that will shift to the north of Miriam. The general
northwestward motion is expected to persist until dissipation. The
track is essentially an update to the prior forecast and is close to
the middle of the guidance envelope near CTCI and HCCA.

Rapid weakening will occur during the next couple of days. The
already vigorous, southwesterly vertical wind shear of 30 to 35 kt
will increase through the next 36 to 48 hours to about 50 kt. In
addition, SSTs, which are currently running around 26.5 C, will
steadily decrease along the forecast track. This will cause Miriam
to weaken to a tropical storm on Saturday and become a remnant low
late Sunday or Monday. The intensity forecast remains in the middle
of the guidance envelope and is close to LGEM through 36 hours,
then remains closer to HWRF.
 
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  01/0300Z 20.1N 141.0W   80 KT  90 MPH
 12H  01/1200Z 21.7N 141.1W   65 KT  75 MPH
 24H  02/0000Z 23.6N 142.1W   50 KT  60 MPH
 36H  02/1200Z 25.6N 143.7W   40 KT  45 MPH
 48H  03/0000Z 27.4N 145.6W   35 KT  40 MPH
 72H  04/0000Z 30.7N 149.3W   30 KT  35 MPH...POST-TROP/REMNT LOW
 96H  05/0000Z...DISSIPATED
 
$$
Forecaster Wroe
 
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